In some fan engines (also known as “propfan” engines), the axis of the fan propeller is parallel to or coaxial with the axis of the gas engine. Typically, in a turbo-shaft, turbo-propeller engine, the axis of one or more propellers will be perpendicular to the axis of the gas engine. In both configurations, the fan or propeller may have a fixed pitch or a variable pitch. If the pitch is variable, the engine may also have a dedicated pitch change mechanism (PCM). The propeller speed (Nx) is proportional to the gas engine power turbine shaft speed (Np) via a pure mechanical gear-train transformation, that is, Nx=Kgb*Np where Kgb is a constant that represents the gear ratio. Controlling the propeller speed, Nx, is equivalent to controlling the power turbine speed, Np. The primary challenge is to coordinate control of the propeller speed (Nx) or the Power Turbine speed (Np) (denoted generically as Nx due to their relationship with each other), the HP shaft speed (N2), and any PCM pitch angle while maintaining a set of active constraints including but not limited to core pressure (Px), exhaust temperature (T), core speed rate (N2dot), and/or torque (Tq) to stay with defined limits, while rejecting external disturbances including but not limited to load change and/or internal known disturbances including but not limited to variable bleed valves and variable stator vanes. The challenge includes two important aspects, one is what control system should be designed to realize the coordinate control objectives, the other is what control references should be scheduled for the control system to follow and achieve the expected control objectives.
There remains a need for a systematic control reference scheduling method to provide coordinate references for a variable pitch fan engine or a turbo-shaft, turbo propeller engine control system.